It has previously been proposed to control the speed of a universal motor by phase control, in that the motor is serially connected to one or more thyristors which are triggered or fired in timed relation to undulations of an a-c supply network so that the relative proportion of the phase of the half waves being applied to the motor can be controlled. Reference is made to German Disclosure Document DT-OS 23 24 174 which discloses a speed control system for an electric drill in which a control network is used including a thyristor. This system is comparatively complex and requires a substantial number of circuit components; additionally, it provides current supply to the motor only during a half wave of the supplied alternating current so that the full power rating of the motor cannot be utilized.